Early molecular events that lead to reproductive death of irradiated mammalian cells cannot be identified unless non-viable cells can be isolated shortly after irradiation. The isolation of non-viable cells by density-gradient electrophoresis is proposed on the basis of findings that electrophoretic mobility correlates with cell death and that the proposed method successfully separates mammalian cells according to electrophoretic mobility. Cultured mouse lymphoma cells, Chinese hamster fibroblasts, and human epitheloid cells will be gamma-irradiated and separated into subpopulations by electrophoresis and tested for reproductive viability. If non-viable subpopulations are isolated, they will be further tested for the correlation of DNA strand breaks, chromosome aberrations, deficient DNA synthesis, and cell surface changes with reproductive lethality due to ionizing radiation.